Whispering gallery mode (WGM) resonators have shown much promise in terms of their versatility and scope in the last number of decades. These microcavities, with inherently small mode volumes (Vm) and high-Q factors, allow for strong light-matter interactions and have become widely used in bio-sensing and nanoparticle detection, temperature, refractive index and pressure sensing quantum optics and electrodynamics, microlaser development, and as a means of exploring optomechanical and nonlinear effects. Despite extensive research on microcavities of various geometries, fabrication methods, and properties, the commonplace bulky experimental apparatus used in microresonator experiments has impeded their successful incorporation into many lab-on-a-chip or miniaturized systems.
FIGS. 5A, 5B, and 5C show an assortment of Prior Art WGM mechanisms. Light travelling inside sphere, strikes glass-air interface at angle of incidence greater than a critical angle, which gives rise to a concept known as total internal reflection (TIR). If a particular microsphere is of good quality, light passed therethrough can undergo multiple reflections, which leads to long photon storage lifetimes, and low mode volume. These are desired characteristics. Specifically, long photon storage lifetimes means it is possible to improve storage of photons due to increased light intensity in a cavity located within the microsphere.
In order to further augment the sensitivity of these WGM systems post-fabrication, the coupling between a given resonator and optical source must be properly managed. With regards to microsphere resonator systems, finely tuned coupling is usually achieved by means of a mechanical or piezoelectric nanometer resolution positioner. However, such devices are difficult to incorporate into miniaturized microsphere or lab-on-a-chip systems that require a tunable coupling mechanism. Other non-mechanical means of realizing coupling regime control have been explored in the past, but had limitations. Consequently, a more efficient coupling mechanism is desired.